Modeling the Liquid Phase Exfoliation of Graphene in Polar and Nonpolar Solvents

Exfoliation is a promising technique to obtain graphene from graphite. The search for suitable exfoliation solvents is currently underway. The quality of the solvents used for spontaneous exfoliation is determined by a simple thermodynamic model. The model shows that the solvation energy of the organic solvents is higher for NMP (-177.37 mJ m-2) than other nonpolar solvents. It also shows that the solvation energy is correlated with sheet deformation and surface excess. Four groups of effective solvents are identified, including amine-, sulfoxide-, halogen-benzene-based solvents, in addition to cyclic structures with the oxygen atom. One can predict and screen potential solvents for spontaneous graphene exfoliation based on the reported mechanism.

Phase-sensitive radar as a tool for measuring firn compaction

Firn compaction models inform mass-balance estimates and paleo-climate reconstructions, but current models introduce key uncertainties. For example, models disagree on the dependence of density and compaction on accumulation rate. Observations of compaction to test these models are rare, partly because in situ methods for measuring englacial strain are time-consuming and expensive. Moreover, shallow measurements may confound strain due to compaction with strain due to ice-sheet flow. We show that phase-sensitive radio-echo sounder (pRES) systems, typically deployed to measure sub-shelf melting or ice-sheet deformation, can be used to measure firn compaction and test firn models. We present two complementary methods for extracting compaction information from pRES data, along with a method for comparing compaction models to pRES observations. The methods make different assumptions about the density structure and vary in their need for independent density measurements. Compaction profiles computed from pRES data collected on three ice rises in West Antarctica are largely consistent with measured densities and a physics-based model. With their minimal logistic requirements, new pRES systems, such as autonomous pRES, could be inexpensively deployed to monitor firn compaction more widely. Existing phase-sensitive radar data may contain compaction information even when surveys targeted other processes.

Numerical and experimental investigation of the effect of process parameters on sheet deformation during the electromagnetic forming of AA6061-T6 alloy

Electromagnetic forming is a high-speed sheet metal forming technique to form metallic sheets by applying magnetic forces. In comparison to the conventional sheet metal forming process, electromagnetic forming is a process with an extremely high velocity and strain rate, which can be effectively used for the forming of certain difficult-to-form metals. During electromagnetic forming, it is important to recognise the effects of process parameters on the deformation and sheet thickness variation of the sheet metal. This research focuses on the development of a numerical model for aluminium alloy (AA6061-T6) to analyse the effects of three process parameters, namely voltage, sheet thickness and number turns of the coils, on the deformation and thickness variation of the sheet. A two-dimensional fully coupled finite-element (FE) model consisting of an electrical circuit, magnetic field and solid mechanics was developed and used to determine the effect of changing magnetic flux and system inductance on sheet deformation. Experiment validation of the results was performed on a 28 KJ electromagnetic forming system. The Taguchi orthogonal array approach was used for the design of experiments using the three input parameters (voltage, sheet thickness and number of turns of the coil). The maximum error between numerical and experimental values for sheet thickness variation was observed to be 4.9 %. Analysis of variance (ANOVA) was performed on the experimental results. Applied voltage and sheet thickness were the significant parameters, while the number of turns of the coil had an insignificant effect on sheet deformation. The contribution ratio of voltage and sheet thickness was 46.21 % and 45.12 % respectively. The sheet deformation from simulations was found to be in good agreement with the experimental results.

EFFECT OF THE PLASTIC STRAIN AND DRAWING QUALITY ON THE FRICTIONAL RESISTANCE OF STEEL SHEETS

The aim of the research presented in this article is to investigate the frictional resistance of steel sheets with different drawing quality. Friction tests have been carried out using the bending under tension (BUT) test which simulates the contact conditions at the rounded edges of the punch and die in sheet metal forming operations. The effect of sheet deformation and temper state on the value of the coefficient of friction has been studied. It was found that increasing the value of elongation of the sheet is associated with an increase in the value of the COF for both friction conditions analysed. The intensity of work hardening, by changing the mechanical properties of the sheet, is a factor that changes contact conditions. The lubricant which is typically used in plastic working provided a reduction of frictional resistance by approximately 3.6-14%, depending on the degree of sheet deformation.

Dynamic Uniform Deformation for Electromagnetic Uniaxial Tension

To compare with quasi-static uniaxial tensioning, researchers designed an electromagnetic uniaxial tension method using a runway coil. However, the requirements to obtain a uniformly deformed sample and the ways the stress changes on the sample using a runway coil have not been studied in the past. In this study, a three-dimensional (3D) sequential coupling method was developed to analyze the factors affecting on-sheet deformation inhomogeneity under electromagnetic uniaxial tension. Two main process parameters, comprising the die type and the relative position of the coil and sheet, were evaluated. Under the optimal parameters, the experiment and simulation both obtained uniformly deformed samples with different discharge conditions, and the simulation method had a high accuracy in modeling the deformation process. The stress state of the sample is approximately unidirectional tensile stress before 240 μs. After 240 μs, the three main stresses showed significant oscillations.

Ultra-Thin Sheet Forming Analyses Considering Size Effects

Based on robust numerical formulations and various material models, finite element (FE) analysis becomes a powerful tool in conventional sheet metal forming process. Unfortunately, the present constitutive equations irrelevant to thickness that describe well conventional sheet deformation modes have difficulties being applied directly to ultra-thin sheet deformation modes. In the present study, a constitutive equation considering size effect is established by introducing a scale factor that represents size effects through thickness and width directions. Uniaxial tensile tests were used to evaluate the scale factor of different thicknesses together with the parameter identification. The developed constitutive equation reveals that thickness is the most important factor effecting on the constitutive relation of ultra-thin sheet. 2D draw forming process of C7035 ultra-thin sheet is analyzed using JSTAMP/NV introducing the developed constitutive equation. The analysis results show that there are obvious differences in the punch forces and loading geometries according to the size effect through thickness direction. Specimen width has slight effect on the flow stress although specimen thickness has strong effect on the flow stress. It is expected that the proposed constitutive equation gives good applicability to FE analysis of micro-scale forming.